1. Field of the Invention
The present invention relates generally to chemical sensing methods and more particularly to chemical sensing methods employing preconcentrators and chromatography columns and more particularly to chemical sensing methods used in portable systems.
2. Description of Related Art
Conventional systems used for the identification of volatile chemicals whether alone or as part of complex mixtures present in trace amounts to high concentrations tend to heavily consume power and are typically bulky and non-portable. Examples of volatile chemicals include chemical warfare agents and explosives and conventional systems are typically used to monitor locations where such chemicals are used, produced or stored. Portable chemical sensing systems typically require the use of compressed gases, thereby limiting their utility.
Frequently used chemical analysis tools consist of gas chromatographs (“GC”) typically provided with a flame ionization detector (“FID”) and thermal conductivity detectors (“TCD”). Mass spectrometers (“MS”) are also used in chemical analysis. Conventionally, these tools are confined to bench top instruments that require a trained operator to transfer vapor or liquid samples to a laboratory for testing. Detectors such as the FID and TCD require a carrier gas to detect the target chemicals tethering the instrument to a gas cylinder and, consequently, usually to a laboratory. For example, the NIOSH method for naphthalene detection uses a GC with FID in which helium is used as a carrier gas.
Some conventional tubular preconcentrators have been used but are generally formed from relatively large one eighth inch (or larger) inside diameter metal tubing 1 stuffed with glass fibers coated in some absorptive material. The tubing 1 is typically wrapped in nichrome wire 3 which heats the tubing when an electrical current is passed through it as shown in FIG. 1a. Such preconcentrators suffer from deficiencies that include the escape of heat from the nichrome wire 3 to the surroundings, the requirement of heating a relatively large thermal mass of the tubing 1 in order to facilitate heat transfer to the inside of the metal tube 1 (causing power drain and time lags) and the hinderance of heat flow at the interior of the tube 2 because of the poor heat conduction pathways of glass fiber matrices, which also hamper the passage of the carrier gas. These deficiencies impede uniform heating of the interior matrix to ensure uniform desorption of target chemicals.